Deadly bird flu sweeps across Hong Kong

More than 170,000 chickens have been culled

HONG KONG, China (CNN) -- The bird flu virus sweeping Hong Kong has been linked to a strain that killed six people in 1997.

Officials say the flu strain, though not identical to the deadly H5N1-type virus, was from the same family and was "more versatile and promiscuous," the territory's South China Morning Post reported on Wednesday.

Authorities will stop all poultry sales across Hong Kong for the day on Friday after finding the virus had spread to six more chicken farms -- bringing the number of known infected farms to nine and the total under quarantine to 24.

More than 170,000 chickens have been culled in the past week since the outbreak was first discovered at a farm in Hong Kong's New Territories region.

A decision on whether to cull a further 700,000 birds in the quarantined farms has not yet been made.

VIDEO CNN's Mike Chinoy reports on the slaughter of chickens in Hong Kong to avoid humans contracting bird flu. Play video (QuickTime, Real or Windows Media)   MORE STORIES Consumers lose taste for chicken     CNN.com Asia More news from our Asia edition

The scientist leading the investigation into the outbreak, Professor Kennedy Shortbridge, told the Post that the Hong Kong government may need to be more aggressive after the virus was unnoticed until two days after birds began dying.

"It is of the Guangdong goose family [of H5N1] but is moving away from the Guangdong virus of 1996 [the parent strain of the 1997 virus]," Shortbridge told the Post.

Hong Kong authorities have increased their surveillance of the disease after a second bird flu outbreak in May.

That outbreak did not affect humans, unlike the deadly 1997 strain which jumped to humans, infecting 18 people, six fatally.

Professor Shortbridge told the paper that the current strain was not a concern to humans but stressed that "very close surveillance of its behavior" was necessary.

He said that if the virus was indeed evolving it could force the government to change its anti-bird flu operations.

Hong Kong's Secretary for Food Lily Yam Kwan Pui-ying downplayed the threat to humans, saying that the current virus had not been completely identified.

"I should add that even if it were H5N1 there are many, many types of H5N1 viruses and we should not jump to conclusions that all H5N1 viruses would affect human beings," Yam said.

Friday's market closure was advised by government authorities in an effort to reduce the spread of the outbreak.